1. Field of the Invention
This invention is related to the coating of metal articles, and more particularly to coating metal articles with aqueous dispersions of vinyl chloride/alkene polymers.
2. The Prior Art
In the manufacture of metal containers, a protective synthetic resin coating is applied to the interior of the container. The synthetic resins which are employed for coating the interior of the metal container are generally resinous materials which are applied in the form of a solution or dispersion in a volatile organic solvent. The wet-coated surface is passed through an oven which hot air is circulated to evaporate the solvent and to cure the coating material to the required hardness. The removal of the organic solvent creates an air pollution problem which many present day communities will not tolerate.
Among the various methods which have been proposed to avoid the use of organic solvents in preparing synthetic resin coatings for metal surfaces is to formulate the coating as an aqueous dispersion.
Aqueous dispersions of vinyl chloride/alkene copolymers have been proposed by the art as coating materials. These copolymers have not found wide commercial application in the food container industry because of the extremely high standards of coating integrity required for food containers, especially in those containers used for packaging carbonated beverages such as beer and soft drinks.
In the packaging of carbonated beverages, the interior coating film is subjected to an acidic liquid under high pressure. Under such demanding conditions, it is extremely critical that any coating applied to the interior of the container be as inert as possible to the packaged contents as well as processing conditions. To date aqueous dispersions of coating materials based on vinyl chloride/alkene copolymers have not wholly met the requirements of the carbonated beverage industry.